The present invention relates to an electrophotographic recording material comprising a conductive substrate, a first photoconductive selenium layer containing halogen disposed on the substrate and a second photoconductive selenium layer containing tellurium disposed on the first photoconductive layer.
Electrophotographic recording materials are used for electrophotographic copying processes which have found wide acceptance in the duplicating art. Such processes are based on the property of the photoconductive material to change its electrical resistance when exposed to an activating radiation.
After a photoconductive layer has been electrically charged and exposed to an activating radiation in a pattern corresponding to an optical image, a latent electrical charge image, which corresponds to the optical image, is produced on the photoconductive layer. At the exposed locations, the conductivity of the photoconductive layer is increased to such an extent that the electrical charge can flow off, at least in part, through the conductive substrate, but in any event the flow off is at a greater extent at the exposed locations than at the unexposed locations. At the unexposed locations, the electrical charge should remain essentially intact, and the pattern of the charge can then be made visible by means of an image powder, a so-called toner. The resulting toner image, if necessary, can then be transferred to paper or some other medium.
Electrophotographically active substances which have been employed include organic as well as inorganic substances. Among the inorganic substances which have been used, selenium, selenium alloys and selenium compounds have gained particular significance. The selenium containing substances play an important role, particularly in their amorphous state, and have found many uses in practice.
The change in electrical conductivity of a photoconductor depends on the intensity and the wavelength of the employed radiation. Within the range of visible light, which is preferred for practical use in electrophotography, for example in office copiers, amorphous selenium exhibits high sensitivity on the blue side, i.e. in the short-wave range, whereas on the red, i.e. in the longwave range, it exhibits a very low sensitivity.
The result is that a red character is reproduced on an electrophotographic plate in the same manner as a black character, which under certain circumstances, particularly with colored masters, may present practical disadvantages, since a black character on a red background--or vice versa--will not be distinguishable from its background and can not, therefore, be made visible. For wavelengths in the infrared range, amorphous selenium is not suitable at all.
In contradistinction to amorphous selenium, crystallized selenium is known to be red sensitive. Thus, the use of crystallized selenium makes possible reproduction involving this part of the visible spectrum. However, the high dark conductivity (dark discharge) of crystallized selenium, i.e. its characteristic of being such a good conductor for electric current while in the unexposed state that a charge applied to its surface cannot be maintained for the length of time required for electrophotographic purposes, discourages its use for such purposes.
Additions to selenium, such as, for example, arsenic or tellurium, are known to broaden the spectral sensitivity of selenium into the longer wave spectral range.
It is also known that by adding halogens to the selenium, an undesirable residual potential which is exhibited by selenium is reduced, which is desirable. On the other hand, too high a halogen content in the selenium again has a disadvantageous influence on its dark discharge.
It may be advisable, under certain circumstances, for the photoconductor of the electrophotographic recording material to be composed of several layers, so that the advantageous characteristics of each individual layer can be utilized simultaneously. Such a recording material is disclosed, for example, in German Pat. No. 941,767 and in DE-OS No. 1,572,375.
In such an arrangement, however, difficulties arise during the manufacturing process with respect to measuring the quantities to be added if one or a plurality of halogens are added to the selenium, for example to the lower layer, in order to reduce the residual potential. Depending on the copying speed, 1 to 3 ppm halogen are already sufficient for this purpose. Higher halogen concentrations lead to the above-mentioned, undesirable high dark discharge which makes the photoconductor unsuitable for purposes of electrophotography. Low halogen concentrations of 1 to 3 ppm halogen, however, can be maintained in practice in a reproduceable manner only with more difficulty. Moreover, the respectively required halogen concentration differs from selenium charge to selenium charge, delivered by producers, so that the required halogen concentration must be set with an accuracy down to 0.1 ppm. Under such conditions, reliable and economical manufacture is hardly possible.